For example
\[\frac{1}{x+5}+\frac{10}{x^2+5x}\rightarrow\frac{1}{x+5}+\frac{10}{x(x+5)}\]
the common Denom. is x(x+5) so all we need to do is multiply the first term by (x/x)
\[\frac{1}{x+5}+\frac{10}{x(x+5)}\rightarrow\frac{1}{x+5}\frac{x}{x}+\frac{10}{x(x+5)}\]\]
\[\frac{x}{x(x+5)}+\frac{10}{x(x+5)}=\frac{5+10}{x(x+5)}=\frac{15}{x(x+5)}\]